(a) Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and, more particularly, to an in-plane-switching (IPS) mode LCD device, which is capable of reducing asymmetry in the leakage light on the screen of the LCD device.
(b) Description of the Related Art
In general, an in-plane-switching (IPS) mode LCD device includes a liquid crystal (LC) layer, a pair of substrates sandwiching therebetween the LC layer, and a pair of polarizing films each attached is with a corresponding one of the substrates on the outer side thereof. In the IPS mode LCD device, the initial orientation of the LC molecules in the LC layer is determined so that the screen of the LCD device represents a black color when no voltage is applied onto the LC layer, and so that the screen represents a white color when a specific voltage is applied onto the LC layer to thereby turn the orientation of the LC molecules therein by about 45 degrees. The IPS mode LCD device generally achieves a higher viewing angle compared to a twisted-nematic (TN) mode LCD device because the rotational direction of the LC molecules is parallel to the substrates in the IPS mode LCD device.
It is known in the IPS mode LCD device that a leakage light appears when the LCD device is observed in an oblique direction upon display of a black color on the screen. Patent Publication JP-A-2002-55341 describes a technique for suppressing the leakage light in the IPS mode LCD device. In this technique, a retardation film or optical compensation film is used for compensating the retardation of the LC layer to thereby suppress the leakage light, the retardation film having a retardation opposite to the retardation of the LC layer and an optical axis slanted to the direction same as the tilting direction of the LC molecules.
In the IPS mode LCD device, although the LC is homogeneously oriented in the direction parallel to the substrates, the LC layer has in fact a pre-tilt angle at each of the boundaries between the same and the orientation film of the TFT (thin-film-transistor) substrate and between the same and the orientation film of the color-filter (CF) substrate. This pre-tilt angle causes the user to observe different amounts of leakage light in the LCD device depending on the viewing angles although the difference itself therebetween is relatively small, because the different viewing angles provide different leakage states.
FIG. 16A shows an example of the orientation of the LC molecules in an IPS mode LCD device having an anti-parallel-oriented LC layer. FIG. 16B shows the orientation of the LC as viewed from the front of the LCD device. As shown in FIG. 16A, the longer optical axes 202 of the LC molecules 201 have a pre-tilt angle θ t11 on the surface of the TFT substrate 102, whereas the longer optical axes 202 of the LC molecules 201 have a pre-tilt angle θ c11 on the surface of the CF substrate 104. The magnitude of the pre-tilt angle θ t11 is equal to the magnitude of the pre-tilt angle θ c11, whereas the rising direction of the LC molecules 201 on the TFT substrate 102 is opposite to the rising direction of the LC molecules 201 on the CF substrate 104.
FIG. 17 shows a luminance viewing cone calculated by a simulation upon display of a black color in the anti-parallel-oriented LCD device. The directions adopted in the luminance viewing cone are such that the Z-axis in FIG. 16A is assumed as the front direction, or at a polar angle of zero degree, the Y-axis in FIG. 16B is assumed at an azimuth angle of zero degree. The viewing cone of FIG. 17 shows the luminance levels of the leakage light as observed upon display of the black color, by using iso-luminance contour lines, for the azimuth angle of zero to 360 degrees and for the polar angle of zero to 80 degrees. In the simulation used for obtaining the luminance viewing cone, the pre-tilt angle is assumed as |θ t11|=|θ c11|=1.5 degrees, whereas the rising direction of the LC molecules is assumed as the positive direction of the X-axis on the TFT substrate and as the negative direction of the X-axis on the CF substrate.
FIG. 18 shows the relationship between the polar angle and the normalized luminance in the directions at azimuth angles of 25/205 degrees (shown by solid line) and at azimuth angles of 65/245 degrees (shown by dotted line). In this figure, the maximum luminance shown in FIG. 17 is used for normalization of the calculated luminances. Assuming that the direction at an azimuth angle of 25 degrees is a plus side of the graph and the direction at an azimuth angle of 205 degrees is a minus side, the peak luminance of the leakage light shown by a solid curve (a) on the plus side is far larger than the peak luminance of the leakage light on the minus side in the direction at the azimuth angles of 25/205 degrees. Assuming that the direction at an azimuth angle of 65 degrees is a plus side of the graph and the direction at an azimuth angle of 245 degrees is a minus side, the peak luminance of the leakage light shown by a dotted curve (b) on the minus side is far greater than, i.e., double, the peak luminance on the plus side in the direction at azimuth angles of 65/245 degrees.
In the anti-parallel orientation of the LC layer, the rising direction of the LC molecules is opposite between the TFT substrate side and the CF substrate side, as shown in FIG. 16A. This causes different observed images of the LC molecules between the cases of the LC panel being obliquely observed from the bottom side and from the top side along the X-axis, if the LC molecules are aligned in the X-axis, as shown in FIG. 16B. As a result, the luminance upon display of a black color is asymmetric between the top-side view and the bottom-side view, as shown in FIGS. 17 and 18, which degrades the overall image quality of the LCD device.
FIG. 19 shows orientations of LC molecules in a splay-oriented mode LCD device. In the splay-oriented mode LCD device, the LC molecules 201 have a pre-tilt angle θ t22 on the TFT substrate and a pre-tilt angle θ c22 on the CF substrate. The orientations of the splay-oriented mode LCD device are such that one of the rising directions of the LC molecules on the TFT substrate and the CF substrate is rotated by 180 degrees in the anti-parallel orientations whereby the rising directions of the LC molecules on the TFT substrate and the CF substrate are the same direction as observed from the front. This configuration of the splay-oriented mode LCD device reduces the difference in the observed image of the LC molecules between the cases of the LC panel being obliquely observed from the top side and the bottom side along the X-axis, thereby reducing the asymmetry. in the luminance-to-viewing angle characteristic between the top side view and the bottom side view. This provides a solution for the top-bottom asymmetry problem involved with the anti-parallel orientation mode LCD device.
FIG. 20 shows a luminance viewing cone upon display of a black color in a splay-oriented mode LCD device, obtained by a simulation. In FIG. 20, as is the case shown in FIG. 17, the Z-axis is assumed at a polar angle of zero degree, the Y-axis is assumed at an azimuth angle of zero degree, wherein the luminance of the leakage light upon display of the black color is shown by iso-luminance contour lines for the azimuth angle of zero to 360 degrees and for the polar angle of zero to 80 degrees. In this simulation, it is assumed that an LCD device has a pre-tilt angle of |θ t22|=|θ c22|=0.5 degrees, and the rising direction of the LC molecules on both the TFT substrate and the CF substrate is along the positive direction of the X-axis.
FIG. 21 shows the relationship between the polar angle and luminance in the directions of the azimuth angles of 25/205 degrees and azimuth angles of 65/245 degrees. In FIG. 21, graph (a) shows the relationship between the polar angle and the normalized luminance in the direction of the azimuth angles of 25/205 degrees, whereas graph (b) shows the relationship between the polar angle and the normalized luminance in the direction of the azimuth angles of 65/245 degrees. For example, the ratio of the peak luminance in the minus side (bottom side) in the azimuth direction of 60/245 degrees to the peak luminance in the plus side (top side) is about 1.35:1. Comparing the graphs of FIG. 21 with the graphs of FIG. 18, it will be understood that the splay-oriented mode LCD device has a better top-bottom symmetry than the anti-parallel mode LCD device, due to the small difference between the peak luminance in the top side and the peak luminance in the bottom side in the splay-orientated mode LCD device.
It should be noted that although the observer scarcely perceives the viewing angle dependency of the leakage light if the leakage light has a higher luminance upon display of a black color, the observer perceives a relatively strong viewing angle dependency of the leakage light if the leakage light itself has a smaller luminance due to, for example, use of an optical compensation film to suppress the total level of the leakage light. This is because the observer can generally perceive a small difference in the suppressed level of the leakage light. That is, the reduction of the leakage light rather degrades the image quality of the LCD device based on the viewing angle dependency of the luminance of the leakage light. Thus, the improvement in the symmetry of the leakage light achieved in the splay-oriented mode LCD device is not sufficient, and it is required to further improve the image quality in the splay-oriented mode LCD device.